1. Field of the Invention
The present invention relates to a process for the preparation of diisocyanates and/or triisocyanates by phosgenation of the corresponding diamines and/or triamines in the gas phase.
2. Description of the Prior Art
The preparation of isocyanates by reaction of amines with phosgene in the gas phase has long been known (cf. Siefken, Annalen 562, 108 (1949). Gas-phase reactions can be carried out in various ways. Nozzles, burners or mixing tubes are used for mixing the starting materials. For the gas-phase phosgenation of diisocyanates, the use of nozzles has been very generally described. These are, as described, for example, in EP-A1-0593334, smooth jet nozzles or concentric feed tubes. Usually, one of the starting materials is sprayed through a centrally arranged nozzle into the stream of the second starting material which flows through the annular space around the nozzle tube at low velocity. The faster-flowing starting material aspirates the slow-flowing starting material, and mixing occurs. After a time or distance dependent on the nozzle diameter and on the difference between the flow velocities of the starting materials, complete mixing of the starting materials is then achieved. The chemical reaction is superposed on the mixing. The gas-phase phosgenation of amines is a reaction whose rate is determined by the mixing of the starting materials. Since the isocyanates formed can undergo secondary reactions with the amines, rapid mixing and an excess of phosgene are necessary for achieving a high selectivity with respect to the desired diisocyanate. Owing to back-mixing processes, the diisocyanate reacts with unreacted diamine from the starting material stream with formation of solid deposits. This results in soiling of the reactor below the mixing zone and in blockages of the reactor.
On an increase of the size of the reactor, which is frequently in the form of a tubular reactor, an increase in the size of the mixing nozzle, which is frequently in the form of a smooth jet nozzle, is also necessary. With the increase in the diameter of the smooth jet nozzle, however, the rate of mixing of the central jet is reduced by the greater diffusion distance required and the danger of back-mixing is increased, which in turn leads to the formation of polymeric impurities and hence caking of solid materials in the reactor.
In British Patent Specification 1165831, the reaction is carried out in a tubular reactor equipped with a mechanical stirrer. The reactor resembles a thin-film evaporator in which the stirrer mixes the gases and at the same time scrapes against the heated walls of the tubular reactor in order thus to prevent a build-up of polymeric material on the tube wall. However, the use of a high-speed stirrer when handling phosgene at about 300° C. requires a high level of safety measures in order to seal the reactor and to mount the stirrer in the highly corrosive medium.
It is therefore an object of the present invention to provide a process for the preparation of diisocyanates and/or triisocyanates in the gas phase, in which the starting materials diamine and phosgene can be mixed more rapidly and better in a reactor without moving internals and in which the formation of polymeric impurities and of caking of the reactor can be avoided.